Construction-Temporary Structures Outline and Problems

PE Review Course – Course – 2016

Construction – Construction – Temporary Structures

Construction – Temporary Structures Outline and Sample Problems Professional Engineering Review Course For Civil Engineers John A. Gambatese, PhD, PE-CA School of Civil and Construction Engineering Oregon State University 201B Kearney Hall Corvallis, OR 97331 (541) 737-8913 [email protected]

March 2016

PE Review Course – 2016

Construction – Temporary Structures

2016 PE Civil Engineering Review Course Construction – Temporary Structures John A. Gambatese, PhD, PE-CA School of Civil and Construction Engineering Oregon State University

INTRODUCTION

This part of the review course covers the Temporary Structures topics within the Construction portion of the breadth (morning) part of the Civil Engineering PE exam. The Construction breadth portion of the exam covers the following topics: Project Planning, Means and Methods, and Site Development, and amounts to approximately 20% of the entire breadth part of the PE exam. The specific Temporary Structures topics covered in the Construction portion of the breadth (morning) part of the exam are: 1. Construction loads 2. Temporary structures and facilities Each of these topics is covered in this review packet. This review packet also covers some of the Temporary Structures topics included in the Construction dep th (afternoon) portion of the exam. The specific Temporary Structures topics covered in the Construction portion of the depth (afternoon) part of the exam are: 1. 2. 3. 4. 5. 6.

Construction loads, codes, and standards Formwork Falsework and scaffolding Shoring and reshoring Bracing and anchorage for stability Temporary support of excavation

This review packet provides an overview of each topic along with the main concepts and formulas for applying the concepts on problems. The sample problems included in this packet provide an opportunity to practice solving problems related to the topics.

REFERENCES AND ADDITIONAL RESOURCES

There are a variety of references available related to temporary structures, some textbooks with example problems and others that are designed for the practicing professional. Some valuable and commonly-used references are also available on-line. The following are suggested references:


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“Design Loads on Structures during Construction,” SEI/ASCE 37-14. American Society of Civil Engineers, Reston, VA, www.asce.org, 2014. th “Formwork for Concrete,” SP-4, 8 Edition. American Concrete Institute, Farmington Hills, MI, www.concrete.org, 2014. “Guide to Formwork for Concrete,” American Concrete Institute, Farmington Hills, MI, www.concrete.org, 2004. (Available in Appendix of ACI SP-4.) “National Design Specification (NDS) for Wood Construction,” ANSI/AF&PA NDS2015. American Forest & Paper Association (AF&PA), Washington, D.C., www.awc.org. th “Steel Construction Manual,” 14 Edition. American Institute of Steel Construction, Inc., Chicago, IL, www.aisc.org. “Standard Practice for Bracing Masonry Walls during Con struction,” Council for Masonry Wall Bracing, Mason Contractors Association of America, Lombard, IL, www.masoncontractors.org. “Occupational Safety and Health Standards for the Construction Industry,” 29 CFR Part 1926, U.S. Department of Labor, Washington, D.C.

CONSTRUCTION LOADING

Temporary structures are structural systems that are used during the construction of a permanent facility. The structures are designed to be used in service for only a limited time and have a limited exposure period to environmental loadings. Examples of temporary structures are: concrete formwork, cofferdams, slurry walls, underpinning, shoring, scaffolds, ladders, and falsework. When in service, temporary structures support both temporary construction loads a nd in some cases the permanent in-service loading. ASCE’s Standard “Design Loads on Structures during Construction” (SEI/ASCE 37-02) provides performance criteria, design loads, load combinations, and safety factors to be used in the analysis and design of structures during their transient stages of construction as well as temporary structures used in construction op erations. Five different types of loading on the temporary structures are addressed: dead load, live load, construction loads, lateral earth pressure, and environmental loads. It is important to also consider the effects on the loads due to the construction methods and sequencing. That is, the loading may change during the construction sequence, and the impact of the changing loads must be taken into consideration when designing the temporary structures. Dead Load (D) The dead load is defined as the weight of all materials of construction incorporated into the permanent structures. It includes construction for which the primary structural system is complete, but which is being used to support construction materials and construction equipment. Dead load does not include construction dead load, CD. Live Load (L) The live load is defined as loads produced by the use or occupancy of a permanent structure that is under construction. This load may vary at different stages of construction, and may be



different than the live loads applied on the completed structure. For bridge structures, it can include impact, longitudinal forces from vehicles, centrifugal forces from vehicles, and wind loads on vehicles. Construction Loads Construction loads are loads imposed on a temporary structure as a result of the construction process. This category of loads is comprised of the following types of loads: 

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Construction dead load (CD): The dead load of the temporary structures.

The weight of personnel and equipment Personnel and equipment loads (CP): (hand tools and light equipment) employed in the construction process. This type of loading can vary significantly during the course of construction. The maximum loads that are likely to be created during the sequence of construction should be used. In addition, either concentrated (point) loads or distributed loads should be used, whichever is more critical. ASCE 37-02 provides a table with values of expected concentrated personnel and equipment loads. Individual personnel load: A concentrated load of 250 lbs (1.1 kN). Includes the o weight of one person plus equipment carried by the person or equipment that can be readily picked up by a single person without assistance. Material loads (CFML and CVML): Fixed Material Loads (CFML) o Load from materials that are fixed in magnitude. Variable Material Loads (CVML) o Load from materials that vary in magnitude during the construction process. Includes materials that are temporarily stockpiled. Material loads can vary between FML and VML during the construction process. o For example, fresh concrete is a VML until it cures when it becomes a FML. Formwork stockpiled and during erection is a VML and is a FML after it is put in place. Materials hauled by heavy equipment are considered part of the equipment load, not a material load. 

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Combined construction loads (personnel, equipment, and materials): When actual values of personnel, equipment, and materials loads are not known, o ASCE 37-02 allows for the use of standard loading based on the level of use for the working surface: 20 psf – Very light duty: sparsely populated with personnel; hand tools; very small amounts of construction materials 25 psf – Light duty: sparsely populated with personnel; hand operated equipment; staging of materials for lightweight construction 50 psf – Medium duty: concentrations of personnel; staging of materials for average construction 75 psf – Heavy duty: material placement by motorized buggies; staging of materials for heavy construction 

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These loads do not include dead load, construction dead load, or fixed material loads.

Reduction in construction loads (personnel, equipment, and materials): o Material loads: No reduction except to the extent that small amounts of material in transit or staging are included in uniformly distributed personnel, equipment, and material loads. o

Personnel and equipment loads (C p): 2 For members having an influence area, AI, of 400 ft or more, when justified by an analysis of the construction operations. 

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C p = Lo [0.25 + (15 / (AI) )] Where:

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C p = reduced design uniformly distributed personnel and 2 equipment load per ft of area supported by the member. Lo = unreduced uniformly distributed personnel and 2 equipment design load per ft of area supported by the member. 2 AI = influence area (ft )

Influence area, AI, normally calculated as: 4x the tributary area for a column 2x tributary area for a beam Equal to the panel area for a two-way slab   

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Reduced load, regardless of the influence area shall not be less than: 50% of the unreduced design load for members supporting one level, or 40% of the unreduced design load for members supporting more than one level, Except where the uniformly distributed personnel and equipment load is 25 psf or less, the reduced load shall not be less than 60% of the unreduced design load, unless justified by an analysis of the construction operations. 

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Personnel and equipment loads on sloping roofs: Reduction factor, R = 1.2 – 0.05F 

Where:  

F = slope of the roof expressed in inches per foot.

R need not exceed 1.0 and shall not be less than 0.6. This reduction may be combined by multiplication with the reduction based on area, but the reduced load shall not be less than 60% of the basic unreduced load.


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Impact loads: Very short term, impact loads must also be included in the design of temporary o structures if they are significant. ASCE 37-02 does not specify a minimum design load requirements for impact loading, except for stating that, “Provision shall be made in the structural design for loads that involve predictable unusual vibration and impact forces.” Horizontal construction load (CH): A minimum amount of horizontal construction load must be applied due to o unavoidable eccentricities and personnel and equipment operations. One of the following minimum horizontal loads shall be applied, whichever gives the greatest structural effects in the direction under consideration: For wheeled vehicles transporting materials, 20% for a single v ehicle or 10% for two or more vehicles of the fully loaded vehicle weight. The force is to be applied in any direction of possible travel, at the running surface. For equipment reactions, the calculated or rated horizontal loads, whichever are greater. 50 lbs per person (0.22 kN/person), applied at the level of the platform in any direction. 2% of the total vertical load, applied in any direction and spatially distributed in proportion to the mass. This load need not be applied concurrently with wind or seismic load. Horizontal construction loads are not a substitute for environmental loads. o 

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Erection and fitting forces (CF): Forces caused by erection (alignment, fitting, bolting, bracing, guying, etc.). o Equipment reactions (CR): Equipment reactions are defined as the loads under heavy equipment as a result of o the weight of equipment, accessories, and load. The rated loading provided by equipment manufacturer should be used. If the rated loading is not available, loads determined by analysis should be used. Required information for calculating equipment loads: o Weight and dimensions of the equipment and accessories Characteristics of the equipment mountings (wheels, tracks, outriggers, etc.) Weight and location of the load. Consider all directions of travel of the equipment. Loads exerted due to starting, stopping, turning, swinging, and lifting (i.e. impact, traction, centrifugal, etc.) Contact pressure under heavy equipment with tracks (e.g., crane, excavator, o loader): Variables: d = contact length of track w = width of each track  

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M = overturning moment due to load, boom/bucket, and other rigging accessories V = total resultant vertical load on track e = eccentricity from center of track to resultant of loads l = length of contact pressure p = track contact pressure

For boom over the front/rear condition: e = M/V 

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When e < d/6 (trapezoidal pressure distribution where l > d): p = (V/wd) [1 ± 6e/d] When e = d/6 (triangular pressure distribution where l = d): pmax = 2V/wd When e > d/6 (triangular pressure distribution where l < d): l = 3[d/2 - e] pmax = 2V/wl = 2V/[3w(d/2 - e)]

For boom over the side condition: Pressure distribution under tracks is rectangular. Use statics analysis to solve for track pressures. Rule-of-thumb: 85%/15% distribution of loading.   

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Contact pressure under heavy equipment with wheels (e.g., loader, truck, crane, excavator): Use rated axle loads from manufacturer or statics analysis to de termine point loads under each wheel. Impact loads from equipment: Increase reaction by 30% to allow for impact, unless other values (either larger or smaller) are recommended by the manufacturer, are required by the authority having jurisdiction, or are justified by analysis. 

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Form Pressure: Formwork is designed for the pressure of the newly-placed concrete along with o any other applied personnel, equipment, material, horizontal, earth pressure, and environmental loads. See the Formwork Loads and Pressure section below for further description of the associated concrete pressures.

Lateral Earth Pressure Earth-retaining structures must be designed to properly support loading due to lateral earth pressure. Lateral earth pressure consists of the horizontal or nearly horizontal resultant of forces per unit area created by soil and water on a vertical or nearly vertical plane of a structure. The pressure is dependent on the soil type, and the type, depth, and shape of the structure. Lateral



earth pressures are covered in greater detail as part of the Geotechnical part of the breadth (morning) exam. Environmental Loads The basic reference for computation of environmental loads is the ASCE 7-05, “Minimum Design Loads for Buildings and Other Structures.” The following environmental loads, as determined using ASCE 7-05 except where modified by ASCE 37-02, are considered in the design of temporary structures: wind, thermal, snow, seismic, rain, and ice .

An importance factor, I = 1.0 is used for all environmental loads on temporary structures, regardless of the importance factor used for the permanent structure.

FORMWORK LOADS AND PRESSURE

The following should be included where applicable when considering the loading and pressure on concrete formwork: 

Vertical loads: Fresh concrete, rebar, and other attachments: 150 pcf for normal weight concrete o Self-weight of the formwork: 3-15 psf o Construction live load o Minimum vertical live load: 50 psf (use 75 psf if motorized carts are used to place concrete) Minimum vertical dead + live load: 100 psf (use 125 psf if motorized carts are used to place concrete) Minimum lateral load for slab forms: 100 plf of slab edge, or 2% of total DL on the form (distributed as a uniform load o per linear foot of slab edge), whichever is greater. o Use minimum wind load based on ASCE 7-05 code requirements if greater. Minimum lateral load for wall forms: 100 plf along top of wall, or 15 psf for wind and accidental eccentric loading, o whichever is greater. o Use minimum wind load based on ASCE 7-05 code requirements if greater. 

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Lateral Pressure of Fresh Concrete

The lateral pressure of fresh concrete on wall and column forms depends on the following:      

density (weight) of the concrete height of the pour (similar to fluids or soils) rate of placement of the fresh concrete temperature of the concrete mix admixtures and cement blends used vibration or other consolidation methods



The basic formula for the maximum lateral pressure on wall and column forms is: p = wh

where: p = lateral pressure (psf) w = unit weight of fresh concrete (pcf) h = depth of fluid or plastic concrete from the top of a placement to the point of consideration in the form (feet)

This formula is used for all placements that all of the following conditions: not Type I cement, unit weight of concrete is not 150 pcf, slump exceeds 4 in, external vibrators are used, retarder or workability admixtures are used, and layers in excess of 4 ft are poured. The maximum formwork pressure can be controlled by the rate of pour and temperature of the concrete (concrete solidifies with time). Also, vibration can create temporary lateral forces locally that are at least 10-20% greater than those occurring with simple spading. It should be noted that in ACI SP-4 and below, the pressure formulas are limited to concrete vibrated internally at the time of placement (not external vibration or revibration). Include additional pressure due to pumping if concrete is pumped into the bottom of the forms (additional pressure to overcome frictional flow resistance). The increase will typically be approximately 50% extra at the pump inlet. Also, Include 100% more pressure than the full liquid head if there are significant restrictions to the flow of the concrete being pumped into the bottom (such as prepacked aggregate, precast elements, large anchorages and embedments, or box-outs). 

Lateral pressure on column forms: Columns are elements with no plan dimension exceeding 6.5 feet. Columns are o small enough to place concrete in the full height of the form in a relatively short period of time; therefore maximum lateral pressures are greater than in wall forms. The maximum pressure for column forms (max. slump = 7 in., internal vibration, o and 4 foot lifts) is given by the following formula:

pmax = CwCc[150 + 9,000(R/T)] where: pmax = maximum lateral pressure (psf) R = rate of placement (feet/hour) o T = temperature of concrete during placing ( F) Cc = chemistry coefficient Cw = unit weight coefficient o

The maximum and minimum limitations on the maximum pressure for column forms are as follows: Minimum pmax = 600Cw Maximum pmax = wh  


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Lateral pressure on wall forms: Walls are vertical structural elements with at least one plane dimension greater o than 6.5 feet. For R < 7 feet/hour and placement height does not exceed 14 feet, maximum o pressure for wall forms (max. slump = 7 in., internal vibration, and 4 foot lifts):

pmax = CwCc[150 + 9,000(R/T)] where: pmax = maximum lateral pressure (psf) R = rate of placement (feet/hour) o T = temperature of concrete during placing ( F) Cc = chemistry coefficient Cw = unit weight coefficient o

For all walls with 7 < R < 15 feet/hour, and for walls with R < 7 and placement height exceeding 14 feet, maximum pressure for wall forms (max. slump = 7 in., internal vibration, and 4 foot lifts): pmax = CwCc[150 + 43,400/T + 2,800(R/T)] where: pmax = maximum lateral pressure (psf) R = rate of placement (feet/hour) o T = temperature of concrete during placing ( F) Cc = chemistry coefficient Cw = unit weight coefficient

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For all walls with R > 15 feet/hour: pmax = wh

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The maximum and minimum limitations on the maximum pressure for wall forms are as follows: Minimum pmax = 600Cw Maximum pmax = wh  

Wall Form Design Procedures

The following procedure is typically used to design formwork for walls and columns (sheathing, studs, wales, and form ties: 1. Determine maximum pressure on the sheathing. a. Use rate of pour, temperature, and form height. 2. Determine sheathing thickness and stud spacing. a. Check sheathing bending, shear, and deflection. 3. Determine stud size and spacing of stud supports (wale spacing). a. Check stud bending, shear, and deflection.



4. Determine wale size and spacing of wale supports (tie spacing). a. Check wale bending, shear, and deflection. 5. Size and space ties. 6. Check bearing stresses. 7. Determine lateral bracing for formwork. Slab Form Design Procedure

The following procedure is typically used to design formwork for slabs (sheathing, joists, stringers, and shores: 1. Determine vertical dead load and live load on the formwork. 2. Determine sheathing thickness and joist spacing. a. Check sheathing bending, shear, and deflection. 3. Determine joist size and spacing of joist supports (stringer spacing). a. Check joist bending, shear, and deflection. 4. Determine stringer size and spacing of stringer supports (shore spacing). a. Check stringer bending, shear, and deflection. 5. Size and space shores. 6. Check bearing stresses. 7. Determine lateral bracing for formwork. Shoring and Reshoring of Multi-Story Buildings

The following tables are example shoring/reshoring sequences and the corresponding loads on the slabs, shores, and reshores. The tables provide the ability to determine the loading in each element as a result of the selected construction sequence. The loadings can then be used to verify that the slabs have sufficient capacity and to design the shores and reshores. The loading values are presented in terms of the weight of the slab, D. The values include: the weight of the slab, an assumed weight of construction live load (0.50D), and an assumed weight of the shores and forms (0.10D). The weight of the reshores is neglected. The tables are also based on the following assumptions: 1. Shores and reshores are infinitely stiff relative to the slabs. 2. Slabs interconnected by shores therefore all deflect equally when a new load is added, and carry a share of the added load in proportion to their relative stiffnesses. 3. Slabs have equal stiffness and added loads are shared equally by the interconnected slabs. 4. The ground floor or other base support is rigid. Also, the sequence of operations for shoring/reshoring successive levels is assumed to be the following: 1. Set up a story of shores and forms, and place the fresh concrete. 2. Remove the reshores at the lowest interconnected level. Remove forms and shores from next story above.



3. Place reshores snugly under the slab just stripped, but without the reshores carrying any load when they are first placed. The reshores should not be placed such that they relieve part of the load on the slab being reshored. Reshores should merely function as a strut to help in distributing the next load applied from a newly placed slab. The slab capacity will depend on the speed of construction as the work progresses up the building (e.g., one floor every 7 days). Based on the speed of construction, the capacity of the slab to support load can be determined based on the extent to which the concrete has cured. This capacity can then be compared to the maximum loading found in the tables in order to verify whether the slabs will have enough capacity to accommodate the planned sequence. Similar tables can be prepared for different shoring/reshoring patterns (e.g., two levels of shoring and two levels of reshoring).



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(Source: “Formwork for Concrete,” 7 Edition, by M.K. Hurd., ACI, SP-4, 2005.)



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SAMPLE PROBLEMS

The following sample problems are provided to give examples of how to determine the loading on temporary structures for various applications. The actual problems on the PE exam may be similar problems or include only a portion o f each sample problem.

1. Determine the maximum design pressure on the formwork for a wall that is 15 feet tall if the o concrete will be placed at a rate R = 10 feet per hour when the temperature is 60 F. Assume normal weight concrete, and Type I cement with a retarding admixture. Determine the maximum design pressure if the wall is only 6 feet tall. 2. A crawler crane will be located on a temporary work platform for lifting material on a project. The total resultant vertical load due to the weight of the crane plus the maximum material load that it will pick on the project will be 83,500 lbs (centered at the midpoint of the tracks), creating an overturning moment of 163,200 lb-ft. The contact area beneath each track on the crane is 19’-0” long and 36 inches wide, and the tracks are located 13’-10” apart (center-to-center of the tracks). Determine the maximum pressure under the crane tracks on the work platform for: a) hoisting the load o ver the front of the crane (boom parallel to the tracks); and b) hoisting the load over the side of the crane (boom perpendicular to the tracks).

Construction-Temporary Structures Outline and Problems

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